The present application is a National Stage Application of International Application No. PCT/FR97/01555, filed Sep. 3, 1997. Further, the present application claims priority under 35 U.S.C. xc2xa7 119 of French Patent Application No. 96/11146 filed on Sep. 12, 1996.
1. Field of the Invention
This invention relates to an energy filter, also called velocity fitter, a transmission electron microscope and associated method for filtering energy
2. Description of Background and Relevant Information
The invention is especially applicable to TEM (Transmission Electron Microscope) or to combined TEM-STEM (Scanning Transmission Electron Microscope) as well as to electron sources. It could be used for specific STEM microscopes.
A notable shortcoming of transmission electron microscopes, during the formation of images or of diffraction diagrams lies in the presence of chromatic aberrations. The latter, essentially due to faulty adjustable electromagnetic lenses of the microscope, are detrimental to contrast and to resolution. Chromatic aberrations can be reduced to a certain extent by applying an electron acceleration voltage that is both high and stable, and by observing very narrow samples.
However, a manner, particularly efficient and accurate, to improve the picture consists in eliminating a portion of the dispersed electrons in a non-elastic way using an energy filter.
The electrons having undergone a given loss of energy may also be employed to form the picture. By selecting a characteristic loss of a type of interaction or of a chemical element, we can obtain a filtered picture providing the mapping of this type of interaction or of this element.
Energy filtering also enables to form the picture of samples that would be too thick to be observed with a conventional transmission electron microscope.
An energy filter usually comprises spatial dispersion means for the electrons of the beam transmitted by the sample in relation to their energy, as well as a filtering slot enabling selection of an energy window. Besides filtering pictures or diffraction diagrams, energy filters are also employed for spectral analysis of energy losses. Energy filters can be installed in an electron microscope either inside the column of the microscope as an integral part of the instrument, or as an accessory below the visualisation screen. We shall find recent reports on several types of filters known in the articles by Bernard Jouffrey:  less than  less than Energy loss spectroscopy for transmission electron microscopy greater than  greater than  in Electron Microscopy in Materials Science, World Scientific, 1991, pp. 363-368 and by Harald Rose and Dieter Krahl:  less than  less than Electron optics of imaging energy filters greater than  greater than , in Energy Filtering Transmission Electron Microscopy, Springer, 1995, pp. 43-55.
For example, the article in the magazine Optik, vol. 96, no4, pp. 163-178 by Uhlemann and Rose, describes a mandolin-type magnetic energy filter.
A parameter determining energy filters is energy dispersion D, expressed in xcexcm/eV: the greater this parameter, the greater the selective power of the filter. In order to increase this dispersion D, various energy filters have been suggested, which cause the electrons of the beam to follow sufficiently long an optical path. Indeed, the dispersion D increases in particular with the length of the distance covered. Thus, in so-called xcexa9 systems, while remaining in a fixed vertical plane, the beam propagating along the optical axis of the system is first deviated laterally, runs then along an optical path substantially parallel to the optical axis in the propagation direction, then is deviated towards the optical axis of the microscope before it is brought back in alignment with its initial direction.
The problem of the filters employed usually lies in their space requirements. Good dispersion D of the filter is indeed obtained by causing the electrons to run a distance over sufficient height. Vertical space requirements of the current filters range generally between 25 and 50 cm, for a dispersion D not exceeding 6 xcexcm/eV.
The European patent application EP-40.538.938 suggested an electron beam instrument provided with an energy selective device. The latter causes the electrons to follow a path in a dispersion plane not containing the optical axis of the instrument. The vertical space requirements of the energy selective device are then reduced considerably for a given path length. In the specific embodiment disclosed in said document (FIG. 3), the energy filter comprises four beam deviating elements located in the dispersion plane, in the respective corners of a substantially rectangular figure which accepts two orthogonal planes of symmetry. The filter also comprises a first deflecting element deviating the beam of the optical axis of the microscope towards one of the deviating elements in the dispersion plane, and a second deflecting element deviating the beam coming from another deviating element in alignment with the optical axis.
The present invention relates to an energy filter capable of producing a wide dispersion D while exhibiting small vertical space requirements, notably increasing the dispersion properties of the filter disclosed in the document EP-0.538.938.
Another object of the invention is such a filter that can be used in conventional electron microscopy at high as well as at low voltage, stigmatic in the first order and affected by small aberrations only.
An additional object of the invention is an energy filter enabling high acceleration voltages.
The invention also relates to a transmission electron microscope provided with an energy filter generating wide dispersion D while exhibiting reasonable vertical space requirements, whereas this microscope can be notably of TEM or TEM-STEM type.
The invention also relates to an energy filtering method for an electron beam propagating along an optical axis, generating wide dispersion over small height along the optical axis, whereas this method can be applied to imaging, diffraction or spectrometry.
To this end, the invention relates to an energy filter receiving during operation an electron beam oriented along an optical axis in a propagation direction. The energy filter comprises:
a deflecting system that deviates in a dispersion plane not containing the optical axis, the beam received along the optical axis, and
a dispersing system that guides the beam sent from the deflecting system on an optical path inscribed in the dispersion plane and returning to the deflecting system, and which generates a spatial dispersion of the electrons of the beam in relation to their energy,
whereby the deflecting system brings back in alignment with the optical axis in the propagation direction the beam coming from the dispersing system.
According to the invention, the deflecting system comprises a single deflecting element ensuring inverse deviations of the beam, whether outgoing or incoming.
The energy filter according to the invention is different with respect to the existing systems in that it comprises a single deflecting element which, both, deviates the beam in a dispersion plane not including the optical axis and provides inverse deviations of the beam, whether outgoing or incoming. The vertical space requirements of this energy filter are therefore reduced considerably, while the latter remains particularly efficient, thus ensuring wide dispersion, small aberrations and other suitable optical properties.
The expressions  less than  less than deflecting system greater than  greater than  and  less than  less than dispersing system greater than  greater than  are generic expressions referring to the main technical effect of each of both systems. It is however extremely difficult to prevent the deflecting system from also causing dispersion, even if the latter is rather reduced. Similarly, the dispersing system generates deflections of the beam that follow energy dispersion.
The outgoing and incoming paths between the deflecting and dispersing systems are generally collinear, although slight discrepancies may appear in relation to one another.
The energy filter may thus provide particularly satisfactory results, notably as regards dispersion D.
The dispersing system should advantageously cause the electron beam to describe a closed curve not surrounding the optical axis.
Thus, the optical path covered by the beam in the deflection plane accepts convexity changes, notably favourable to the limitation of second order aberrations.
Preferably, the deflecting and dispersing systems are symmetrical with respect to a plane of symmetry containing the axis and with respect to the dispersion plane.
This configuration of the systems enables to obtain correct stigmatism and achromatism properties.
According to an advantageous embodiment of the energy filter, the dispersion plane is perpendicular to the optical axis.
According to a preferred embodiment of the dispersing system of the. energy filter according to the invention, the latter comprises:
a first deviating element, which receives the beam coming from the deflecting system and which deviates the latter along an incoming direction, and
a second deviating element, which receives from the first deviating element the beam along the incoming direction, causes it to follow a circular path in the dispersion plane and sends it back towards the first deviating element along an outgoing direction,
whereas the first deviating element receives the beam coming from the second deviating element along the outgoing direction and deviates it towards the deflecting system.
It is then advantageous that the first deviating element is arranged between the axis and the second deviating element, whereas the second element comprises an external aperture in which the first element is installed partially.
In this particular embodiment, the dispersing system causes the electron beam to describe a closed curve not surrounding the optical axis.
According to a preferred embodiment of this lay-out:
the first element comprises a pair of polar parts parallel to the dispersion plane, each being hexagonal in shape and comprising a larger base perpendicular to the axis facing the deflecting system, two perpendicular sides connected at right angle to the larger base and parallel to the plane of symmetry, two oblique sides, respectively connected to the perpendicular sides and a smaller base; opposite and parallel to this larger base and connected to oblique sides, and,
the second element comprises a pair of polar parts, respectively coplanar with the polar parts of the first element, each crown-shaped, comprising a centre arranged in the plane of symmetry and whose external aperture reaches inside the crown through a passage facing the smaller base of the polar parts of the first element and is delineated laterally by two sides, respectively, facing the oblique sides of the polar parts of the first element.
Advantageously, the deflecting and dispersing systems comprising magnetic sectors which comprise, each, a pair of opposite polar parts, separated by an air-gap, connected to actuation and control means in order to create in each air-gap a requested magnetic field, whereby the magnetic fields are uniform in each air-gap.
The invention also relates to a transmission electron microscope provided with an energy filter according to the invention.
The invention also relates to a method for energy filtering of an electron beam propagating along an optical axis in a propagation direction. In this method:
the beam is guided on an optical path substantially inscribed in a dispersion plane not including the optical axis in order to generate dispersion of the beam electrons in relation to their energy,
the beam is re-directed in alignment with the axis in the propagation direction, and
an energy window is selected spatially.
According to the invention, the beam is caused to follow in the dispersion plane incoming and outgoing paths which are collinear as well as of opposite directions.